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Simon D. M. White - One of the best experts on this subject based on the ideXlab platform.
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exploring the non linear density field in the millennium simulations with tessellations i the probability distribution function
Monthly Notices of the Royal Astronomical Society, 2013Co-Authors: Biswajit Pandey, Simon D. M. White, Volker Springel, Raul E AnguloAbstract:We use the Delaunay Tessellation Field Estimator (DTFE) to study the one-point density distribution functions of the Millennium (MS) and Millennium-II (MS-II) simulations. The DTFE technique is based directly on the particle positions, without requiring any type of smoothing or analysis grid, thereby providing high sensitivity to all non-linear structures resolved by the simulations. In order to identify the detailed origin of the shape of the one-point density probability distribution function (PDF), we decompose the simulation particles according to the mass of their host FoF Halos, and examine the contributions of different halo mass ranges to the global density PDF. We model the one-point distribution of the FoF Halos in each halo mass bin with a set of Monte Carlo realizations of idealized NFW dark matter Halos, finding that this reproduces the measurements from th e N-body simulations reasonably well, except for a small excess present in simulation result s. This excess increases with increasing halo mass. We show that its origin lies in substructure, which becomes progressively more abundant and better resolved in more massive dark matter Halos. We demonstrate that the high density tail of the one-point distribution functio n in less massive Halos is severely affected by the gravitational softening length and the mass resolution. In particular, we find these two parameters to be more important for an accurate measurement of the density PDF than the simulated volume. Combining our results from individual halo mass bins we find that the part of the one-point density PDF originating from collapsed Halos can nevertheless be quite well described by a simple superposition of a set of NFW Halos with the expected cosmological abundance over the resolved mass range. The transition region to the low-density unbound material is however not well captured by such an analytic halo model.
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stellar discs in aquarius dark matter haloes
Monthly Notices of the Royal Astronomical Society, 2012Co-Authors: Jackson Debuhr, Simon D. M. WhiteAbstract:We investigate the gravitational interactions between live stellar disks and their dark matter Halos, using CDM haloes similar in mass to that of the Milky Way taken from the Aquarius Project. We introduce the stellar disks by rst allowing the haloes to respond to the inuence of a growing rigid disk potential from z = 1:3 to z = 1:0. The rigid potential is then replaced with star particles which evolve self-consistently with the dark matter particles until z = 0:0. Regardless of the initial orientation of the disk, the inner parts of the haloes contract and change from prolate to oblate as the disk grows to its full size. When the disk normal is initially aligned with the major axis of the halo at z = 1:3, the length of the major axis contracts and becomes the minor axis by z = 1:0. Six out of the eight disks in our main set of simulations form bars, and ve of the six bars experience a buckling instability that results in a sudden jump in the vertical stellar velocity dispersion and an accompanying drop in the m = 2 Fourier amplitude of the disk surface density. The bars are not destroyed by the buckling but continue to grow until the present day. Bars are largely absent when the disk mass is reduced by a factor of two or more; the relative disk-to-halo mass is therefore a primary factor in bar formation and evolution. A subset of the disks is warped at the outskirts and contains prominent non-coplanar material with a ring-like structure. Many disks reorient by large angles between z = 1 and z = 0, following a coherent reorientation of their inner haloes. Larger reorientations produce more strongly warped disks, suggesting a tight link between the two phenomena. The origins of bars and warps appear independent: some disks with strong bars show little disturbances at the outskirts, while the disks with the weakest bars show severe warps.
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stellar disks in aquarius dark matter haloes
arXiv: Cosmology and Nongalactic Astrophysics, 2012Co-Authors: Jackson Debuhr, Simon D. M. WhiteAbstract:We investigate the gravitational interactions between live stellar disks and their dark matter Halos, using LCDM haloes similar in mass to that of the Milky Way taken from the Aquarius Project. We introduce the stellar disks by first allowing the haloes to respond to the influence of a growing rigid disk potential from z = 1.3 to z = 1.0. The rigid potential is then replaced with star particles which evolve self-consistently with the dark matter particles until z = 0.0. Regardless of the initial orientation of the disk, the inner parts of the haloes contract and change from prolate to oblate as the disk grows to its full size. When the disk normal is initially aligned with the major axis of the halo at z=1.3, the length of the major axis contracts and becomes the minor axis by z=1.0. Six out of the eight disks in our main set of simulations form bars, and five of the six bars experience a buckling instability that results in a sudden jump in the vertical stellar velocity dispersion and an accompanying drop in the m=2 Fourier amplitude of the disk surface density. The bars are not destroyed by the buckling but continue to grow until the present day. Bars are largely absent when the disk mass is reduced by a factor of two or more; the relative disk-to-halo mass is therefore a primary factor in bar formation and evolution. A subset of the disks is warped at the outskirts and contains prominent non-coplanar material with a ring-like structure. Many disks reorient by large angles between z=1 and z=0, following a coherent reorientation of their inner haloes. Larger reorientations produce more strongly warped disks, suggesting a tight link between the two phenomena. The origins of bars and warps appear independent: some disks with strong bars show no disturbances at the outskirts, while the disks with the weakest bars show severe warps.
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substructure in the stellar Halos of the aquarius simulations
The Astrophysical Journal, 2011Co-Authors: Amina Helmi, Carlos S Frenk, Simon D. M. White, Shaun Cole, Andrew P Cooper, Julio F NavarroAbstract:We characterize the substructure in the simulated stellar Halos of Cooper et al. which were formed by the disruption of satellite galaxies within the cosmological N-body simulations of galactic Halos of the Aquarius project. These stellar Halos exhibit a wealth of tidal features: broad overdensities and very narrow faint streams akin to those observed around the Milky Way. The substructures are distributed anisotropically on the sky, a characteristic that should become apparent in the next generation of photometric surveys. The normalized RMS of the density of stars on the sky appears to be systematically larger for our Halos compared with the value estimated for the Milky Way from main-sequence turnoff stars in the Sloan Digital Sky Survey. We show that this is likely to be due in part to contamination by faint QSOs and redder main-sequence stars, and might suggest that ~10% of the Milky Way halo stars have formed in situ.
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there s no place like home statistics of milky way mass dark matter Halos
arXiv: Cosmology and Nongalactic Astrophysics, 2009Co-Authors: Michael Boylankolchin, Volker Springel, Simon D. M. White, Adrian JenkinsAbstract:We present an analysis of the distribution of structural properties for Milky Way-mass Halos in the Millennium-II Simulation (MS-II). This simulation of structure formation within the standard LCDM cosmology contains thousands of Milky Way-mass Halos and has sufficient resolution to properly resolve many subHalos per host. It thus provides a major improvement in the statistical power available to explore the distribution of internal structure for Halos of this mass. In addition, the MS-II contains lower resolution versions of the Aquarius Project Halos, allowing us to compare our results to simulations of six Halos at a much higher resolution. We study the distributions of mass assembly histories, of subhalo mass functions and accretion times, and of merger and stripping histories for subHalos capable of impacting disks at the centers of Halos. We show that subhalo abundances are not well-described by Poisson statistics at low mass, but rather are dominated by intrinsic scatter. Using the masses of subHalos at infall and the abundance-matching assumption, there is less than a 10% chance that a Milky Way halo with M_vir =10^12 M_sun will host two galaxies as bright as the Magellanic Clouds. This probability rises to ~25% for a halo with M_vir=2.5 x 10^12 M_sun. The statistics relevant for disk heating are very sensitive to the mass range that is considered relevant. Mergers with infall mass : redshift zero virial mass greater than 1:30 could well impact a central galactic disk and are a near inevitability since z=2, whereas only half of all Halos have had a merger with infall mass : redshift zero virial mass greater than 1:10 over this same period.
Stefan Gottlober - One of the best experts on this subject based on the ideXlab platform.
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multidark simulations the story of dark matter halo concentrations and density profiles
Monthly Notices of the Royal Astronomical Society, 2016Co-Authors: Anatoly Klypin, Stefan Gottlober, Gustavo Yepes, Francisco PradaAbstract:Accurately predicting structural properties of dark matter Halos is one of the fundamental goals of modern cosmology. We use the new suite of MultiDark cosmological simulations to study the evolution of dark matter halo density profiles, concentrations, and velocity anisotropies. The MultiDark simulations cover a large range of masses 1e10-1e15Msun and volumes upto 50Gpc**3. The total number of dark matter Halos in all the simulations exceeds 60 billion. We find that in order to understand the structure of dark matter Halos and to make ~1% accurate predictions for density profiles, one needs to realize that halo concentration is more complex than the traditional ratio of the virial radius to the core radius in the NFW profile. For massive Halos the averge density profile is far from the NFW shape and the concentration is defined by both the core radius and the shape parameter alpha in the Einasto approximation. Combining results from different redshifts, masses and cosmologies, we show that Halos progress through three stages of evolution. (1) They start as rare density peaks that experience very fast and nearly radial infall. This radial infall brings mass closer to the center producing a high concentrated halo. Here, the halo concentration increases with the increasing halo mass and the concentration is defined by the alpha parameter with nearly constant core radius. Later Halos slide into (2) the plateau regime where the accretion becomes less radial, but frequent mergers still affect even the central region. Now the concentration does not depend on halo mass. (3) Once the rate of accretion slows down, Halos move into the domain of declining concentration-mass relation because new accretion piles up mass close to the virial radius while the core radius is staying constant. We provide accurate analytical fits to the numerical results for halo density profiles and concentrations.
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the large scale bias of dark matter Halos numerical calibration and model tests
The Astrophysical Journal, 2010Co-Authors: Jeremy L Tinker, Anatoly Klypin, Andrey V Kravtsov, Gustavo Yepes, Brant E Robertson, Michael S Warren, Stefan GottloberAbstract:We measure the clustering of dark matter Halos in a large set of collisionless cosmological simulations of the flatCDM cosmology. Halos are identified using the spherical over density algorithm, which finds the mass around isolated peaks in the density field such that the m ean density istimes the background. We calibrate fitting functions for the large scale bias that are adaptable to any value ofwe examine. We find a � 6% scatter about our best fit bias relation. Our fitting functi ons couple to the halo mass functions of Tinker et. al. (2008) such that bias of all dark matter is normalized to unity. We demonstrate that the bias of massive, rare Halos is higher than that predicted in the modified ellip soidal collapse model of Sheth, Mo, & Tormen (2001), and approaches the predictions of the spherical collapse model for the rarest Halos. Halo bias results based on friends-of-friends Halos identified with linking l ength 0.2 are systematically lower than for Halos with the canonical � = 200 overdensity by � 10%. In contrast to our previous results on the mass function, we find that the universal bias function evolves very weakly with redshift, if at all. We use our numerical results, both for the mass function and the bias relation, to test the peak- background split model for halo bias. We find that the peak-background split achieves a reasonable agreement with the numerical results, but � 20% residuals remain, both at high and low masses. Subject headings:cosmology:theory — methods:numerical — large scale structure of the universe
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the large scale bias of dark matter Halos numerical calibration and model tests
arXiv: Cosmology and Nongalactic Astrophysics, 2010Co-Authors: Jeremy L Tinker, Anatoly Klypin, Andrey V Kravtsov, Gustavo Yepes, Brant E Robertson, Michael S Warren, Stefan GottloberAbstract:We measure the clustering of dark matter Halos in a large set of collisionless cosmological simulations of the flat LCDM cosmology. Halos are identified using the spherical overdensity algorithm, which finds the mass around isolated peaks in the density field such that the mean density is Delta times the background. We calibrate fitting functions for the large scale bias that are adaptable to any value of Delta we examine. We find a ~6% scatter about our best fit bias relation. Our fitting functions couple to the halo mass functions of Tinker et. al. (2008) such that bias of all dark matter is normalized to unity. We demonstrate that the bias of massive, rare Halos is higher than that predicted in the modified ellipsoidal collapse model of Sheth, Mo, & Tormen (2001), and approaches the predictions of the spherical collapse model for the rarest Halos. Halo bias results based on friends-of-friends Halos identified with linking length 0.2 are systematically lower than for Halos with the canonical Delta=200 overdensity by ~10%. In contrast to our previous results on the mass function, we find that the universal bias function evolves very weakly with redshift, if at all. We use our numerical results, both for the mass function and the bias relation, to test the peak-background split model for halo bias. We find that the peak-background split achieves a reasonable agreement with the numerical results, but ~20% residuals remain, both at high and low masses.
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the dependence on environment of cold dark matter halo properties
The Astrophysical Journal, 2005Co-Authors: Vladimir Avilareese, Pedro Colin, Stefan Gottlober, C Firmani, Christian MaulbetschAbstract:A series of high-resolution ΛCDM cosmological N-body simulations are used to study the properties of galaxy-size dark Halos as a function of global environment. We analyze Halos in three types of environment: "cluster" (cluster Halos and their surroundings), "void" (large regions with density contrasts -0.85), and "field" (Halos not contained within larger Halos). We find that Halos in clusters have a median spin parameter ~1.3 times lower, a minor-to-major axial ratio ~1.2 times lower (more spherical), and a less aligned internal angular momentum than Halos in voids and the field. For masses 5 × 1011 h-1 M☉, Halos in cluster regions are on average ~30%-40% more concentrated and have ~2 times higher central densities than Halos in voids. While for Halos in cluster regions the concentration parameters decrease on average with mass with a slope of ~0.1, for Halos in voids these concentrations do not seem to change with mass. When comparing only parent Halos from the samples, the differences are less pronounced but still significant. We obtain also the maximum circular velocity-mass and rms velocity-mass relations. These relations are shallower and more scattered for Halos in clusters than in voids, and for a given circular velocity or rms velocity, the mass is smaller at z = 1 than at z = 0 for all environments. At z = 1, the differences in the halo properties with environment almost disappear, suggesting that the differences were established mainly after z ~ 1. The Halos in the cluster regions undergo more dramatic changes than those in the field or the voids. The differences in halo properties with environment are due to (1) the dependence of halo formation time on global environment and (2) local effects such as tidal stripping and the tumultuous histories that Halos suffer in high-density regions. We calculate seminumerical models of disk galaxy evolution using Halos with the concentrations and spin parameters found for the different environments. For a given disk mass, the galaxy disks have higher surface density, larger maximum circular velocity and secular bulge-to-disk ratio, lower gas fraction, and are redder as one goes from void to cluster environments. Although all these trends agree with observations, the latter tend to show more differences, suggesting that physical ingredients not considered here, such as misalignment of angular momentum, halo triaxiality, merging, ram pressure stripping, harassment, etc., play an important role for galaxy evolution, especially in high-density environments.
Y P Jing - One of the best experts on this subject based on the ideXlab platform.
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mass and redshift dependence of dark halo structure
The Astrophysical Journal, 2003Co-Authors: H J Mo, Y P Jing, D H Zhao, Gerhard BornerAbstract:Using a combination of N-body simulations with different resolutions, we study in detail how the concentrations of cold dark matter Halos depend on halo mass at different redshifts. We confirm that halo concentrations at the present time depend strongly on halo mass, but our results also show marked differences from the predictions of some early empirical models. Our main results are that the mass dependence of the concentrations becomes weaker at higher redshifts and that at z greater than or similar to 3, Halos of mass greater than 10(11) h(-1) M-circle dot all have a similar median concentration, c similar to 3.5. While the median concentrations of low-mass Halos grow significantly with time, those of massive Halos change very little with redshift. These results are quantitatively in good agreement with the empirical model proposed by Zhao et al., which shows that Halos in the early fast accretion phase all have similar concentrations.
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intrinsic correlation of halo ellipticity and its implications for large scale weak lensing surveys
arXiv: Astrophysics, 2002Co-Authors: Y P JingAbstract:We use a large set of state-of-the-art cosmological N-body simulations [512^3 particles] to study the intrinsic ellipticity correlation functions of Halos. With the simulations of different resolutions, we find that the ellipticity correlations converge once the Halos have more than 160 members. For Halos with fewer members, the correlations are underestimated, and the underestimation amounts to a factor of 2 when the Halos have only 20 particles. After correcting for the resolution effects, we show that the ellipticity correlations of Halos in the bigger box (L=300 mpc) agree very well with those obtained in the smaller box (L=100 mpc). Combining these results from the different simulation boxes, we present accurate fitting formulae for the ellipticity correlation function c_{11}(r) and for the projected correlation functions Sigma_{11}(r_p) and Sigma_{22}(r_p) over three orders of magnitude in halo mass. The latter two functions are useful for predicting the contribution of the intrinsic correlations to deep lensing surveys. With reasonable assumptions for the redshift distribution of galaxies and for the mass of galaxies, we find that the intrinsic ellipticity correlation can contribute significantly not only to shallow surveys but also to deep surveys. Our results indicate that previous similar studies significantly underestimated this contribution for their limited simulation resolutions.
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the density profile of equilibrium and nonequilibrium dark matter Halos
The Astrophysical Journal, 2000Co-Authors: Y P JingAbstract:We study the diversity of the density pro—les of dark matter Halos based on a large set of high- resolution cosmological simulations of 2563 particles. The cosmological models include four scale-free models and three representative cold dark matter models. The simulations have good force resolution. In each cosmological model, there are about 400 massive Halos that have more than 104 particles within the virial radius. Our unbiased selection of all massive Halos enables us to quantify how well the bulk of dark matter Halos can be described by the Navarro, Frenk, & White (NFW) pro—le, which was estab- lished for equilibrium Halos. We —nd that about 70% of the Halos can be —tted by the NFW pro—le with a —tting residual in universes. This percentage is higher in the low-density cosmo- dvi max \ 0.3 ) 0 \ 1 logical models of The rest of the Halos exhibit larger deviations from the NFW pro—le for more ) 0 \ 0.3. signi—cant internal substructures. There is a considerable amount of variation in the density pro—le even among the Halos that can be —tted by the NFW pro—le (i.e., The distribution of the pro—le dvi max \ 0.30). parameter, the concentration c, can be described well by a lognormal function with the mean value c6 slightly smaller (15%) than the NFW result and the dispersion, in ln c, of about 0.25. More virialized p c , Halos with have a mean value in better agreement with the NFW result, and their disper- dvi max \ 0.15 c6 sion, is also slightly smaller (about 0.2). Our results can alleviate some of the con—icts found recently p c , between the theoretical NFW pro—le and observational results. Implications for theoretical and obser- vational studies of galaxy formation are discussed. Subject headings: cosmology: theorydark mattergalaxies: formation ¨ large-scale structure of universemethods: n-body simulations
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the density profile of equilibrium and non equilibrium dark matter Halos
arXiv: Astrophysics, 1999Co-Authors: Y P JingAbstract:We study the diversity of the density profiles of dark matter Halos based on a large set of high-resolution cosmological simulations of 256^3 particles. The cosmological models include four scale-free models and three representative cold dark matter models. The simulations have good force resolution, and there are about 400 massive Halos with more than 10^4 particles within the virial radius in each cosmological model. Our unbiased selection of all massive Halos enables to quantify how well the bulk of dark matter Halos can be described by the Navarro, Frenk & White (NFW) profile which was established for equilibrium Halos. We find that about seventy percent of the Halos can be fitted by the NFW profile with a fitting residual dvi_{max} less than 30% in Omega_0=1 universes. This percentage is higher in lower density cosmological models. The rest of the Halos exhibits larger deviations from the NFW profile for more significant internal substructures. There is a considerable amount of variation in the density profile even for the Halos which can be fitted by the NFW profile (i.e. dvi_{max}<0.30). The distribution of the profile parameter, the concentration $c$, can be well described by a lognormal function with the mean value \bar c slightly smaller (15%) than the NFW result and the dispersion \sigma_c in \ln c about 0.25. The more virialized Halos with dvi_{max}<0.15 have the mean value \bar c in good agreement with the NFW result and a slightly smaller dispersion \sigma_c (about 0.2). Our results can alleviate some of the conflicts found recently between the theoretical NFW profile and observational results. Implications for theoretical and observational studies of galaxy formation are discussed.
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accurate fitting formula for the two point correlation function of dark matter Halos
The Astrophysical Journal, 1998Co-Authors: Y P JingAbstract:An accurate fitting formula is reported for the two-point correlation function ? -->hh(r;M) of dark matter Halos in hierarchical clustering models. It is valid for the linearly clustering regime, and its accuracy is about 10% in ? -->hh(r;M) for the Halos with mass M > (10 -->?2-10 -->?3)M -->*, where M -->* is the characteristic nonlinear mass. The result is found on the basis of a careful analysis for a large set of scale-free simulations with 2563 particles. The fitting formula has a weak explicit dependence on the index n of the initial power spectrum but can be equally well applied to the cold dark matter (CDM) cosmological models if the effective index neff${r eff}$ -->$t SUBgt {r eff}t/SUBgt $ --> of the CDM power spectrum at the scale of the halo mass replaces the index n. The formula agrees with the analytical formula of Mo & White for massive Halos with M > M -->*, but the Mo & White formula significantly underpredicts ? -->hh(r;M) for the less massive Halos. The difference between the fitting and the analytical formulae amounts to a factor 2 in ? -->hh(r;M) for M=0.01M -->*. One of the most interesting applications of this fitting formula would be the clustering of galaxies, since the majority of Halos hosting galaxies satisfies MM -->*.
Julio F Navarro - One of the best experts on this subject based on the ideXlab platform.
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substructure in the stellar Halos of the aquarius simulations
The Astrophysical Journal, 2011Co-Authors: Amina Helmi, Carlos S Frenk, Simon D. M. White, Shaun Cole, Andrew P Cooper, Julio F NavarroAbstract:We characterize the substructure in the simulated stellar Halos of Cooper et al. which were formed by the disruption of satellite galaxies within the cosmological N-body simulations of galactic Halos of the Aquarius project. These stellar Halos exhibit a wealth of tidal features: broad overdensities and very narrow faint streams akin to those observed around the Milky Way. The substructures are distributed anisotropically on the sky, a characteristic that should become apparent in the next generation of photometric surveys. The normalized RMS of the density of stars on the sky appears to be systematically larger for our Halos compared with the value estimated for the Milky Way from main-sequence turnoff stars in the Sloan Digital Sky Survey. We show that this is likely to be due in part to contamination by faint QSOs and redder main-sequence stars, and might suggest that ~10% of the Milky Way halo stars have formed in situ.
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the aquarius project the subhaloes of galactic haloes
Monthly Notices of the Royal Astronomical Society, 2008Co-Authors: Volker Springel, Amina Helmi, Carlos S Frenk, Julio F Navarro, Aaron D. Ludlow, Adrian Jenkins, Mark Vogelsberger, Jie Wang, Simon D. M. WhiteAbstract:We have performed the largest ever particle simulation of a Milky Way sized dark matter halo, and present the most comprehensive convergence study for an individual dark matter halo carried out thus far. We have also simulated a sample of six ultrahighly resolved Milky Way sized haloes, allowing us to estimate the halo-to-halo scatter in substructure statistics. In our largest simulation, we resolve nearly 300 000 gravitationally bound subhaloes within the virialized region of the halo. Simulations of the same object differing in mass resolution by factors of up to 1800 accurately reproduce the largest subhaloes with the same mass, maximum circular velocity and position, and yield good convergence for the abundance and internal properties of dark matter substructures. We detect up to four generations of subhaloes within subhaloes, but contrary to recent claims, we find less substructure in subhaloes than in the main halo when regions of equal mean overdensity are compared. The overall substructure mass fraction is much lower in subhaloes than in the main halo. Extrapolating the main halo's subhalo mass spectrum down to an Earth mass, we predict the mass fraction in substructure to be well below 3 per cent within 100 kpc, and to be below 0.1 per cent within the solar circle. The inner density profiles of subhaloes show no sign of converging to a fixed asymptotic slope and are well fitted by gently curving profiles of Einasto form. The mean concentrations of isolated haloes are accurately described by the fitting formula of Neto et al. down to maximum circular velocities of 1.5 km s(-1), an extrapolation over some five orders of magnitude in mass. However, at equal maximum circular velocity, subhaloes are more concentrated than field haloes, with a characteristic density that is typically similar to 2.6 times larger and increases with decreasing distance from halo centre.
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the aquarius project the subHalos of galactic Halos
arXiv: Astrophysics, 2008Co-Authors: Volker Springel, Amina Helmi, Carlos S Frenk, Julio F Navarro, Aaron D. Ludlow, Adrian Jenkins, Mark Vogelsberger, Jie Wang, Simon D. M. WhiteAbstract:We have performed the largest ever particle simulation of a Milky Way-sized dark matter halo, and present the most comprehensive convergence study for an individual dark matter halo carried out thus far. We have also simulated a sample of 6 ultra-highly resolved Milky-way sized Halos, allowing us to estimate the halo-to-halo scatter in substructure statistics. In our largest simulation, we resolve nearly 300,000 gravitationally bound subHalos within the virialized region of the halo. Simulations of the same object differing in mass resolution by factors up to 1800 accurately reproduce the largest subHalos with the same mass, maximum circular velocity and position, and yield good convergence for the abundance and internal properties of dark matter substructures. We detect up to four generations of subHalos within subHalos, but contrary to recent claims, we find less substructure in subHalos than in the main halo when regions of equal mean overdensity are compared. The overall substructure mass fraction is much lower in subHalos than in the main halo. Extrapolating the main halo's subhalo mass spectrum down to an Earth mass, we predict the mass fraction in substructure to be well below 3% within 100 kpc, and to be below 0.1% within the Solar Circle. The inner density profiles of subHalos show no sign of converging to a fixed asymptotic slope and are well fit by gently curving profiles of Einasto form. The mean concentrations of isolated Halos are accurately described by the fitting formula of Neto et al. down to maximum circular velocities of 1.5 km/s, an extrapolation over some 5 orders of magnitude in mass. However, at equal maximum circular velocity, subHalos are more concentrated than field Halos, with a characteristic density that is typically ~2.6 times larger and increases towards the halo centre.
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The Structure of Cold Dark Matter Halos
Cambridge University Press (CUP), 1995Co-Authors: Julio F NavarroAbstract:High resolution N-body simulations show that the density profiles of dark matter Halos formed in the standard CDM cosmogony can be fit accurately by scaling a simple “universal” profile. Regardless of their mass, Halos are nearly isothermal over a large range in radius, but significantly shallower than r–2 near the center and steeper than r–2 in the outer regions. The characteristic overdensity of a halo correlates strongly with halo mass in a manner consistent with the mass dependence of the epoch of halo formation. Matching the shape of the rotation curves of disk galaxies with this halo structure requires (i) disk mass-to-light ratios to increase systematically with luminosity, (ii) halo circular velocities to be systematically lower than the disk rotation speed, and (iii) that the masses of Halos surrounding bright galaxies depend only weakly on galaxy luminosity. This offers an attractive explanation for the puzzling lack of correlation between luminosity and dynamics in observed samples of binary galaxies and of satellite companions of bright spiral galaxies, suggesting that the structure of dark matter Halos surrounding bright spirals is similar to that of cold dark matter Halos.
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The Structure of Cold Dark Matter Halos
The Astrophysical Journal, 1995Co-Authors: Julio F Navarro, Carlos S Frenk, Simon D. M. WhiteAbstract:We use N-body simulations to investigate the structure of dark Halos in the standard Cold Dark Matter cosmogony. Halos are excised from simulations of cosmologically representative regions and are resimulated individually at high resolution. We study objects with masses ranging from those of dwarf galaxy Halos to those of rich galaxy clusters. The spherically averaged density profiles of all our Halos can be fit over two decades in radius by scaling a simple ``universal'' profile. The characteristic overdensity of a halo, or equivalently its concentration, correlates strongly with halo mass in a way which reflects the mass dependence of the epoch of halo formation. Halo profiles are approximately isothermal over a large range in radii, but are significantly shallower than $r^{-2}$ near the center and steeper than $r^{-2}$ near the virial radius. Matching the observed rotation curves of disk galaxies requires disk mass-to-light ratios to increase systematically with luminosity. Further, it suggests that the Halos of bright galaxies depend only weakly on galaxy luminosity and have circular velocities significantly lower than the disk rotation speed. This may explain why luminosity and dynamics are uncorrelated in observed samples of binary galaxies and of satellite/spiral systems. For galaxy clusters, our halo models are consistent both with the presence of giant arcs and with the observed structure of the intracluster medium, and they suggest a simple explanation for the disparate estimates of cluster core radii found by previous authors. Our results also highlight two shortcomings of the CDM model. CDM Halos are too concentrated to be consistent with the halo parameters inferred for dwarf irregulars, and the predicted abundance of galaxy Halos is larger than the observed abundance of galaxies.
Anatoly Klypin - One of the best experts on this subject based on the ideXlab platform.
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multidark simulations the story of dark matter halo concentrations and density profiles
Monthly Notices of the Royal Astronomical Society, 2016Co-Authors: Anatoly Klypin, Stefan Gottlober, Gustavo Yepes, Francisco PradaAbstract:Accurately predicting structural properties of dark matter Halos is one of the fundamental goals of modern cosmology. We use the new suite of MultiDark cosmological simulations to study the evolution of dark matter halo density profiles, concentrations, and velocity anisotropies. The MultiDark simulations cover a large range of masses 1e10-1e15Msun and volumes upto 50Gpc**3. The total number of dark matter Halos in all the simulations exceeds 60 billion. We find that in order to understand the structure of dark matter Halos and to make ~1% accurate predictions for density profiles, one needs to realize that halo concentration is more complex than the traditional ratio of the virial radius to the core radius in the NFW profile. For massive Halos the averge density profile is far from the NFW shape and the concentration is defined by both the core radius and the shape parameter alpha in the Einasto approximation. Combining results from different redshifts, masses and cosmologies, we show that Halos progress through three stages of evolution. (1) They start as rare density peaks that experience very fast and nearly radial infall. This radial infall brings mass closer to the center producing a high concentrated halo. Here, the halo concentration increases with the increasing halo mass and the concentration is defined by the alpha parameter with nearly constant core radius. Later Halos slide into (2) the plateau regime where the accretion becomes less radial, but frequent mergers still affect even the central region. Now the concentration does not depend on halo mass. (3) Once the rate of accretion slows down, Halos move into the domain of declining concentration-mass relation because new accretion piles up mass close to the virial radius while the core radius is staying constant. We provide accurate analytical fits to the numerical results for halo density profiles and concentrations.
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dark matter Halos in the standard cosmological model results from the bolshoi simulation
The Astrophysical Journal, 2011Co-Authors: Anatoly Klypin, Sebastian Trujillogomez, Joel R. PrimackAbstract:Lambda Cold Dark Matter (?CDM) is now the standard theory of structure formation in the universe. We present the first results from the new Bolshoi dissipationless cosmological ?CDM simulation that uses cosmological parameters favored by current observations. The Bolshoi simulation was run in a volume 250 h ?1?Mpc on a side using ~8 billion particles with mass and force resolution adequate to follow subHalos down to the completeness limit of V circ = 50?km?s?1 maximum circular velocity. Using merger trees derived from analysis of 180 stored time steps we find the circular velocities of satellites before they fall into their host Halos. Using excellent statistics of Halos and subHalos (~10 million at every moment and ~50 million over the whole history) we present accurate approximations for statistics such as the halo mass function, the concentrations for distinct Halos and subHalos, the abundance of Halos as a function of their circular velocity, and the abundance and the spatial distribution of subHalos. We find that at high redshifts the concentration falls to a minimum value of about 4.0 and then rises for higher values of halo mass?a new result. We present approximations for the velocity and mass functions of distinct Halos as a function of redshift. We find that while the Sheth-Tormen (ST) approximation for the mass function of Halos found by spherical overdensity is quite accurate at low redshifts, the ST formula overpredicts the abundance of Halos by nearly an order of magnitude by z = 10. We find that the number of subHalos scales with the circular velocity of the host halo as V 1/2 host, and that subHalos have nearly the same radial distribution as dark matter particles at radii 0.3-2 times the host halo virial radius. The subhalo velocity function N(> V sub) scales as V ?3 circ. Combining the results of Bolshoi and Via Lactea-II simulations, we find that inside the virial radius of Halos with the number of satellites is N(> V sub) = (V sub/58 km s?1)?3 for satellite circular velocities in the range 4 km s?1 < V sub < 150 km s?1.
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Halos and galaxies in the standard cosmological model: results from the Bolshoi simulation
arXiv: Cosmology and Nongalactic Astrophysics, 2011Co-Authors: Anatoly Klypin, Sebastian Trujillo-gomez, Joel R. PrimackAbstract:We present the first results from the new Bolshoi N-body cosmological LCDM simulation that uses cosmological parameters favored by current observations. The Bolshoi simulation was done in a volume 250Mpc on a side using 8billion particles with mass and force resolution adequate to follow subHalos down to a completeness limit of Vcirc=50km/ s circular velocity. Using excellent statistics of Halos and subHalos (10M at every moment and 50M over the whole history) we present accurate approximations for statistics such as the halo mass function, the concentrations for distinct Halos and subHalos, abundance of Halos as function of their circular velocity, the abundance and the spatial distribution of subHalos. We find that at high redshifts the concentration falls to a minimum of about 3.8 and then rises slightly for higher values of halo mass. We find that while the Sheth-Tormen approximation for the mass function of Halos found by spherical overdensity is accurate at low redshifts, it over-predicts the abundance of Halos by nearly an order of magnitude by z=10. We find that the number of subHalos scales with the circular velocity of the host halo as Vhost**0.5, and that subHalos have nearly the same radial distribution as dark matter particles at radii 0.3-2 times the host halo virial radius. The subhalo velocity function n(>V) behaves as V**(-3). We give normalization of this relation for different masses and redshifts. Finally, we use an abundance-matching procedure to assign r-band luminosities to dark matter Halos as a function of halo Vcirc, and find that the luminosity-velocity relation is in remarkably good agreement with the observed Tully-Fisher relation for galaxies in the range 50-200km/s.
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the large scale bias of dark matter Halos numerical calibration and model tests
The Astrophysical Journal, 2010Co-Authors: Jeremy L Tinker, Anatoly Klypin, Andrey V Kravtsov, Gustavo Yepes, Brant E Robertson, Michael S Warren, Stefan GottloberAbstract:We measure the clustering of dark matter Halos in a large set of collisionless cosmological simulations of the flatCDM cosmology. Halos are identified using the spherical over density algorithm, which finds the mass around isolated peaks in the density field such that the m ean density istimes the background. We calibrate fitting functions for the large scale bias that are adaptable to any value ofwe examine. We find a � 6% scatter about our best fit bias relation. Our fitting functi ons couple to the halo mass functions of Tinker et. al. (2008) such that bias of all dark matter is normalized to unity. We demonstrate that the bias of massive, rare Halos is higher than that predicted in the modified ellip soidal collapse model of Sheth, Mo, & Tormen (2001), and approaches the predictions of the spherical collapse model for the rarest Halos. Halo bias results based on friends-of-friends Halos identified with linking l ength 0.2 are systematically lower than for Halos with the canonical � = 200 overdensity by � 10%. In contrast to our previous results on the mass function, we find that the universal bias function evolves very weakly with redshift, if at all. We use our numerical results, both for the mass function and the bias relation, to test the peak- background split model for halo bias. We find that the peak-background split achieves a reasonable agreement with the numerical results, but � 20% residuals remain, both at high and low masses. Subject headings:cosmology:theory — methods:numerical — large scale structure of the universe
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the large scale bias of dark matter Halos numerical calibration and model tests
arXiv: Cosmology and Nongalactic Astrophysics, 2010Co-Authors: Jeremy L Tinker, Anatoly Klypin, Andrey V Kravtsov, Gustavo Yepes, Brant E Robertson, Michael S Warren, Stefan GottloberAbstract:We measure the clustering of dark matter Halos in a large set of collisionless cosmological simulations of the flat LCDM cosmology. Halos are identified using the spherical overdensity algorithm, which finds the mass around isolated peaks in the density field such that the mean density is Delta times the background. We calibrate fitting functions for the large scale bias that are adaptable to any value of Delta we examine. We find a ~6% scatter about our best fit bias relation. Our fitting functions couple to the halo mass functions of Tinker et. al. (2008) such that bias of all dark matter is normalized to unity. We demonstrate that the bias of massive, rare Halos is higher than that predicted in the modified ellipsoidal collapse model of Sheth, Mo, & Tormen (2001), and approaches the predictions of the spherical collapse model for the rarest Halos. Halo bias results based on friends-of-friends Halos identified with linking length 0.2 are systematically lower than for Halos with the canonical Delta=200 overdensity by ~10%. In contrast to our previous results on the mass function, we find that the universal bias function evolves very weakly with redshift, if at all. We use our numerical results, both for the mass function and the bias relation, to test the peak-background split model for halo bias. We find that the peak-background split achieves a reasonable agreement with the numerical results, but ~20% residuals remain, both at high and low masses.
